Lithium secondary batteries and other nonaqueous electrolyte secondary batteries are smaller and lighter and have a higher energy density and output density than conventional batteries and as a result are preferentially used as power sources for consumer electronic devices and as vehicle drive power sources. These types of batteries are typically fabricated by housing a nonaqueous electrolyte and an electrode assembly provided with a positive electrode and a negative electrode in a battery case. In addition, the battery (the assembled article) after fabrication is subjected to a charging process under prescribed conditions to adjust it to an actually usable state.
During this charging process, a portion of the nonaqueous electrolyte is reductively degraded at the negative electrode to form, on the surface of the negative electrode active material, a coat that originates from the nonaqueous electrolyte. This coat can then inhibit the reductive degradation of the nonaqueous electrolyte during subsequent charge-discharge cycles. Methods are also known in which an additive (typically a compound that, through degradation at or below the degradation potential of the nonaqueous electrolyte, can form a coat on the surface of the negative electrode active material) is added to the battery in advance in order to make this coat more stable. For example, a nonaqueous electrolyte secondary battery that contains an (oxalato)borate-type compound (e.g., lithium bis(oxalato)borate) is disclosed in Patent Literature 1.